diff --git a/common/common/segments.py b/common/common/segments.py
index 234ca58..9634d32 100644
--- a/common/common/segments.py
+++ b/common/common/segments.py
@@ -13,6 +13,7 @@ import shutil
 import sys
 from collections import namedtuple
 from contextlib import closing
+from tempfile import TemporaryFile
 
 import gevent
 from gevent import subprocess
@@ -286,9 +287,12 @@ def ffmpeg_cut_segment(segment, cut_start=None, cut_end=None):
 	return subprocess.Popen(args, stdout=subprocess.PIPE)
 
 
-def ffmpeg_cut_stdin(cut_start, duration, encode_args):
+def ffmpeg_cut_stdin(output_file, cut_start, duration, encode_args):
 	"""Return a Popen object which is ffmpeg cutting from stdin.
-	This is used when doing a full cut."""
+	This is used when doing a full cut.
+	Note the explicit output file object instead of using a pipe,
+	because most video formats require a seekable file.
+	"""
 	args = [
 		'ffmpeg',
 		'-hide_banner', '-loglevel', 'error', # suppress noisy output
@@ -296,11 +300,18 @@ def ffmpeg_cut_stdin(cut_start, duration, encode_args):
 		'-ss', cut_start,
 		'-t', duration,
 	] + list(encode_args) + [
-		'-', # output to stdout
+		# We want ffmpeg to write to our tempfile, which is its stdout.
+		# However, it assumes that '-' means the output is not seekable.
+		# We trick it into understanding that its stdout is seekable by
+		# telling it to write to the fd via its /proc/self filename.
+		'/proc/self/fd/1',
+		# But of course, that file "already exists", so we need to give it
+		# permission to "overwrite" it.
+		'-y',
 	]
 	args = map(str, args)
 	logging.info("Running full cut with args: {}".format(" ".join(args)))
-	return subprocess.Popen(args, stdin=subprocess.PIPE, stdout=subprocess.PIPE)
+	return subprocess.Popen(args, stdin=subprocess.PIPE, stdout=output_file)
 
 
 def read_chunks(fileobj, chunk_size=16*1024):
@@ -384,20 +395,6 @@ def fast_cut_segments(segments, start, end):
 					yield chunk
 
 
-def feed_input(segments, pipe):
-	"""Write each segment's data into the given pipe in order.
-	This is used to provide input to ffmpeg in a full cut."""
-	for segment in segments:
-		with open(segment.path) as f:
-			try:
-				shutil.copyfileobj(f, pipe)
-			except OSError as e:
-				# ignore EPIPE, as this just means the end cut meant we didn't need all input
-				if e.errno != errno.EPIPE:
-					raise
-	pipe.close()
-
-
 def full_cut_segments(segments, start, end, encode_args):
 	# how far into the first segment to begin
 	cut_start = max(0, (start - segments[0].start).total_seconds())
@@ -405,21 +402,33 @@ def full_cut_segments(segments, start, end, encode_args):
 	duration = (end - start).total_seconds()
 
 	ffmpeg = None
-	input_feeder = None
 	try:
-		ffmpeg = ffmpeg_cut_stdin(cut_start, duration, encode_args)
-		input_feeder = gevent.spawn(feed_input, segments, ffmpeg.stdin)
-		# stream the output until it is closed
-		for chunk in read_chunks(ffmpeg.stdout):
-			yield chunk
+		# Most ffmpeg output formats require a seekable file.
+		# For the same reason, it's not safe to begin uploading until ffmpeg
+		# has finished. We create a temporary file for this.
+		tempfile = TemporaryFile()
+		ffmpeg = ffmpeg_cut_stdin(tempfile, cut_start, duration, encode_args)
+
+		# stream the input
+		for segment in segments:
+			with open(segment.path) as f:
+				try:
+					shutil.copyfileobj(f, ffmpeg.stdin)
+				except OSError as e:
+					# ignore EPIPE, as this just means the end cut meant we didn't need all input
+					if e.errno != errno.EPIPE:
+						raise
+		ffmpeg.stdin.close()
+
 		# check if any errors occurred in input writing, or if ffmpeg exited non-success.
 		if ffmpeg.wait() != 0:
 			raise Exception("Error while streaming cut: ffmpeg exited {}".format(ffmpeg.returncode))
-		input_feeder.get() # re-raise any errors from feed_input()
+
+		# Now actually yield the resulting file
+		for chunk in read_chunks(tempfile):
+			yield chunk
 	finally:
 		# if something goes wrong, try to clean up ignoring errors
-		if input_feeder is not None:
-			input_feeder.kill()
 		if ffmpeg is not None and ffmpeg.poll() is None:
 			for action in (ffmpeg.kill, ffmpeg.stdin.close, ffmpeg.stdout.close):
 				try: